The present invention relates to low porosity coatings, and more particularly, to a method and apparatus for applying an internal, low porosity, vitreous coating to tanks and process equipment.
Glass, or enamel internally coated tanks and process equipment have been in wide use for many years where smooth corrosion resistant internal surfaces are desired, or required. Such equipment is particularly adapted to use in the chemical, food, and beverage processing industries where it is widely used as reactors or tanks, as well as many types of storage, aging and mixing equipment.
The present method and apparatus utilizes the basic coating method described in U.S. Pat. No. 3,788,874 issued to Crandall et al. The method involves applying a low porosity glass coat to an article by maintaining the article at a temperature at least as high as the fusion point of the glass while depositing particles of glass on the article at a rate no greater than the rate at which the particles fuse to the article. While the prinicple and the method of the patent are sound, the method has not enjoyed widespread commercial use because of shortcomings in the technology needed to implement the method. The present invention seeks to remedy these shortcomings and provide for the commercialization of the Crandall et al method.
The teachings of the U.S. Pat. No. 3,788,874 Patent are incorporated herein by reference. The general method covered by the patent is useful in the application of both ground and cover coats. Some of the advantages accruing from the method include: low porosity coating, improved heat transfer, reduced number of heat cycles required to obtain continuous coating, and coatings of increased density may be obtained.
In accord with the present invention, cylindrical vessels are internally glass coated by placing the vessel in a furnace, heating and rotating the vessel as particulate glass is distributed and fused on the internal surface. The vessel is subsequently cooled to produce a vessel having an integral, internal glass lining.
Various means have been proposed to rotate articles within furnaces. For example, U.S. Pat. No. 1,498,515 describes a process of coating articles placed in a heated rotating drum; U.S. Pat. No. 3,044,893 describes a method of coating hot billets by rolling them in a supply of powdered glass; U.S. Pat. No. 3,230,105 teaches coating heated vessels with plastic or metal powder by rolling; and, U.S. Pat. No. 3,484,266 teaches glass coating of tubular items by rotating the items with a supply of powdered glass and subsequently firing. It will be noted that processes that involve high temperature coatings, typically, separate the heating and rolling steps. The reason for this is that at glass firing temperatures, (1500.degree.-1700.degree. F.), metallic substrates, typically iron or steel, are softened and are distorted by a rolling process carried out at glass firing temperatures.